Wave-absorbing and heat-insulating integrated SiC fiber rigid foam and preparation method thereof

Abstract

The invention provides wave-absorbing and heat-insulating integrated SiC fiber rigid foam and a preparation method thereof, polycarbosilane is used as a main precursor, tetrahydrofuran and ethanol are used as a solvent system, polyvinylpyrrolidone is used as a spinning aid, aluminum nitrate nonahydrate is doped to serve as an aluminum source, and the conductivity and component uniformity of a precursor spinning solution are regulated and controlled. A continuous and uniform precursor fiber membrane is prepared through an electrostatic spinning technology, and the SiC fiber rigid foam which is stable in structure and excellent in performance is obtained through thermocuring and high-temperature ceramic treatment. The introduction of aluminum promotes the generation of a graphite carbon phase and the formation of Al-O-C and Al-O-Si structures, and improves the dielectric loss capability and thermal insulation performance of the material. Test results show that the material has high dielectric constant, low thermal conductivity and good compressive property, and is suitable for the fields of high-temperature electromagnetic shielding, thermal insulation protection and the like. The method is simple and convenient in preparation process, and the obtained material is adjustable in performance and has a good engineering application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of structural and functional integrated ceramic materials, and particularly relates to a wave-absorbing and heat-insulating integrated SiC fiber hard foam and a preparation method thereof. BACKGROUND

[0002] The urgent needs of advanced equipment for stealth technology and extreme environment thermal protection materials make functional materials develop towards the direction of wave-absorbing, heat-insulating and structure-carrying integration. At present, electromagnetic wave absorbing materials mainly focus on carbon-based materials (such as graphene, carbon nanotubes) and magnetic metal filler systems. These materials achieve good wave absorption capacity through dielectric loss or magnetic loss, but they generally have defects such as poor thermal stability, insufficient structural rigidity, low strength and high density, which are difficult to serve stably in high-temperature or load environments for a long time.

[0003] Existing related research has made certain progress in improving the flexibility, windability and impact resistance of materials, and has application potential in the fields of electrically insulating films and flexible heating sheets. However, the research core of such flexible ceramic materials focuses on "flexibility" and "deformability", and there is a lack of systematic solutions for the overall structural rigidity, carrying capacity and strength performance under high-temperature service. In composite structures, there are still problems such as low shear strength and weak structural support, which are difficult to meet the engineering application requirements in high-pressure and high-load environments.

[0004] Now there is also a method of adding a reinforcing coating to the outer wall of the SiC foam to improve its compression resistance, but this method relies on a complex impregnation and resintering process, and can only improve the local mechanical properties, and cannot fundamentally enhance the overall structural rigidity of the foam. At the same time, it also does not consider its electromagnetic performance and thermal function, and the application range is still limited.

[0005] There are also existing methods related to the preparation of high-purity α-Al2O3 ceramic fibers. Although such materials have good thermal stability and insulating properties, they generally have the defects of high brittleness and poor impact resistance. In this method, even a second phase material (such as SiO2, B2O3) needs to be introduced to improve its toughness, but the introduction of the second phase may adversely affect its high-temperature performance, exposing the natural limitations of single-component ceramic fibers in structural performance regulation.

[0006] For bulk ceramic materials, there is also a problem of insufficient mechanical properties. Existing research has pointed out that although porous or foam ceramic materials (such as SiC foam) used in wave-absorbing and heat-insulating scenarios have excellent thermal stability and low thermal conductivity, with a thermal conductivity as low as 0.05-0.2 W / m·K, their mechanical strength still cannot meet the structural requirements, with a compression strength of only about 1.44 MPa, which is much lower than the required strength of structural protection materials. Such materials are prone to brittle fracture under high temperature or load, and have poor reliability, which limits their application space in the field of multifunctional integrated materials.

[0007] Therefore, in the prior art system, the flexible ceramic material is soft, the bulk foam ceramic is brittle, and the single-component oxide ceramic has low performance, so it is difficult to balance structural strength, thermal stability and wave absorption performance.

[0008] Although the method of preparing SiC nanofibers by polymer precursor conversion (PDCs) has been widely used in the field of functional ceramics, in the process of electrospinning, due to the low conductivity of PCS itself, problems such as unstable Taylor cone, uneven fiber thickness, even fiber rupture and bead phenomenon often occur in the process of spinning. At the same time, during the high-temperature ceramicization process of pure PCS, the obtained fiber structure is loose, the overall formability is poor although the porosity is high, and the bonding force between the fibers is weak. In addition, the dielectric performance of unmodified SiC fibers in the high-frequency electromagnetic wave band is limited, and the wave absorption performance is insufficient, which cannot meet the comprehensive needs of infrared and electromagnetic compatibility wave absorption and heat insulation integrated materials. SUMMARY

[0009] 1. Technical problems to be solved: Develop a SiC-based nanofiber system with dense structure, good heat treatment adaptability and controllable doping to realize the regulation of fiber connection and density at the microscale, improve the overall rigidity and mechanical support performance of the material at the macro level, and balance the multifunctional properties such as wave absorption and heat insulation.

[0010] 2. Technical solutions: In order to solve the above problems, the application provides a preparation method of wave-absorbing and heat-insulating integrated SiC fiber hard foam, comprising the following steps: Step S01: preparation of precursor spinning solution: mix and stir ethanol and aluminum nitrate nonahydrate until completely dissolved, then add tetrahydrofuran and polyvinylpyrrolidone and stir until completely dissolved; then add polycarbosilane and continue stirring to obtain a uniform precursor spinning solution; Step S02: electrospinning: the precursor spinning solution is loaded into a syringe and electrospun through a needle to obtain an electrospun fiber sheet; Step S03: heat curing treatment: the obtained electrospun fiber sheet is cured to obtain a precursor fiber sheet; Step S04: lamination and pressure: cut several precursor fiber sheets to the same size, stack them to form a fiber mat of a certain thickness, cover a high-purity graphite plate or ceramic plate on the upper surface of the fiber mat, and apply a constant pressure; Step S05: ceramicization treatment: place the fiber mat in a nitrogen atmosphere, heat to 1200-1500℃, and cool after holding to complete the preparation of the SiC fiber hard foam.

[0011] In step S01, in every 10-15 mL precursor spinning solution, ethanol is 2-5 mL, aluminum nitrate nonahydrate is 0.005-0.02 g, polyvinylpyrrolidone is 0.5-2 g, polycarbosilane is 1-3 g, and the rest is tetrahydrofuran.

[0012] In step S02, the voltage of electrostatic spinning is 10-16.5 kV, the injection speed is 3-5 mL / h, the collection distance is 10-20 cm, and the collector rotation speed is 5-500 rad / min.

[0013] The needle type is one of G20, G21, G22 and G24.

[0014] In step S03, the obtained electrostatic spinning fiber sheet is cured at 180-220°C for 1-3 hours.

[0015] In step S04, a constant external pressure of 20-40 Pa is applied to the precursor fiber sheet.

[0016] In step S05, the temperature is raised at 2-8°C / min, and the temperature is kept for 1-3 hours.

[0017] The application also provides an integrated SiC fiber hard foam prepared by the preparation method.

[0018] 3. Beneficial effects: The application introduces an aluminum-containing electrolyte (aluminum nitrate nonahydrate) into a SiC precursor system, and constructs an aluminum-containing modified fiber structure by electrostatic spinning and a PDCs method. In the sintering process, the aluminum element can promote the ordering and graphitization of the carbon phase interface structure, form an Al-O-Si or Al-O-C bridging structure, and improve the connectivity between fibers and the overall stability. The method significantly improves the overall rigidity and mechanical support ability of the material while maintaining the lightweight and porous characteristics of the material, breaking through the bottleneck of the insufficient structural performance of traditional flexible wave-absorbing materials. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a flow chart of the preparation method of the application.

[0020] Figure 2 are the compressive stress-strain curves of examples 1-4 after high-temperature heat treatment in a nitrogen atmosphere; Figure 3 are the X-band reflection loss 3D graphs of examples 1-4 after high-temperature heat treatment in a nitrogen atmosphere, wherein Figure 3 (a) represents when no aluminum nitrate nonahydrate is doped; Figure 3 (b) represents when 0.005 g of aluminum nitrate nonahydrate is doped; Figure 3 (c) represents when 0.01 g of aluminum nitrate nonahydrate is doped;Figure 3 (d) represents when 0.02 g of aluminum nitrate nonahydrate is doped, all the above are in 10-15 mL of precursor spinning solution. DETAILED DESCRIPTION

[0021] The application will be described in detail below with reference to the accompanying drawings and examples.

[0022] The application introduces an aluminum-containing electrolyte (aluminum nitrate nonahydrate) into the PCS spinning precursor solution, and by adjusting the solution system conductivity and component ratio, the spinning performance is improved from the solution stage, and the formation of stable interface structures such as Al-O-Si or Al-O-C in the heat treatment process is guided, so as to improve the mechanical properties and electromagnetic response ability of ceramic fibers. Polyvinylpyrrolidone (PVP) is introduced into the system as a spinning aid, which not only enhances the viscosity of the solution, but also provides dispersion in the precursor. The specific method is as follows: a preparation method of a SiC fiber hard foam doped with an aluminum-containing electrolyte, as shown in FIG. 1, comprises the following steps: Figure 1 Step S01: solution preparation: mix 2-5 mL of ethanol with 0.02 g of aluminum nitrate nonahydrate and stir until completely dissolved, then add tetrahydrofuran; add 0.5-2 g of polyvinylpyrrolidone (PVP) to the obtained solution and stir until completely dissolved; add PCS: add 1-3 g of polycarbosilane (PCS) and continue stirring for 24-48 hours to obtain a uniform precursor spinning solution.

[0023] Step S02: electrospinning: load the precursor spinning solution into a syringe and perform electrospinning through one of the G20, G21, G22, and G24 type needles, with a voltage of 10-16.5 kV, an injection speed of 3-5 mL / h, a collection distance of 10-20 cm, and a collector rotation speed of 5-500 rad / min, to form an electrospun fiber sheet.

[0024] Step S03: heat curing treatment: place the obtained fiber under the condition of 180-220℃ for 1-3 hours to stabilize the structure, to obtain a precursor fiber sheet.

[0025] Step S04: lamination and pressure application: cut the cured precursor fiber sheet into a uniform size and stack it to form a fiber mat with a thickness of 2-4 cm, and apply a constant external pressure of 20-40 Pa to the fiber mat. The uniform size can be customized.

[0026] Step S05: ceramicization treatment: place the fiber mat in a nitrogen atmosphere, heat it to 1200-1500℃ at a rate of 2-8℃ / min, and cool it after holding for 1-3 hours, to complete the preparation of the SiC fiber hard foam.

[0027] Example 1 ​A method for preparing a SiC fiber rigid foam doped with aluminum nitrate nonahydrate. Specifically comprising: mixing 2 mL of anhydrous ethanol with 9 mL of tetrahydrofuran, then adding 0.5 g of polyvinylpyrrolidone (PVP) to fully dissolve, and then adding 1 g of polycarbosilane (PCS), and continuously stirring for 36 hours to obtain a precursor spinning solution. The solution is formed into an electrospinning fiber sheet under the following electrospinning conditions: a voltage of 16.5 kV, an injection speed of 3 mL / h, a collection distance of 15 cm, a needle type of G22, and a collector rotation speed of 300 rad / min. The obtained electrospinning fiber sheet is cured at 220°C for 1 hour to obtain a precursor fiber sheet, the precursor fiber sheet is cut and stacked to obtain a fiber felt with a thickness of 3 cm, and a pressure of 25 Pa is applied, and the fiber felt is heat treated at 1500°C at a heating rate of 2°C / min in a nitrogen atmosphere for 2 hours to complete the ceramicization. The obtained SiC fiber rigid foam has a compact overall structure, XRD analysis shows that the main peak is an amorphous peak of SiO2, Raman spectrum shows that the degree of graphitization is low, the dielectric loss is small, and the wave absorption performance is limited.

[0028] Example 2 A SiC fiber rigid foam doped with 0.005 g of aluminum nitrate nonahydrate. Specifically comprising: mixing 3 mL of anhydrous ethanol with 8 mL of tetrahydrofuran, then adding 1 g of polyvinylpyrrolidone (PVP) to fully dissolve, and then adding 3 g of polycarbosilane (PCS), and continuously stirring for 24 hours to obtain a precursor spinning solution. The solution is formed into an electrospinning fiber sheet under the following electrospinning conditions: a voltage of 12 kV, an injection speed of 3 mL / h, a collection distance of 15 cm, a needle type of G21, and a collector rotation speed of 200 rad / min. The obtained electrospinning fiber sheet is cured at 180°C for 2 hours to obtain a precursor fiber sheet, the precursor fiber sheet is cut and stacked to obtain a fiber felt with a thickness of 4 cm, and a pressure of 20 Pa is applied, and the fiber felt is heat treated at 1200°C at a heating rate of 5°C / min in a nitrogen atmosphere for 3 hours to complete the ceramicization. The obtained SiC fiber rigid foam has a more loose structure than the sample without doping aluminum element, the graphite phase is enhanced, the D / G intensity ratio in the Raman spectrum is significantly improved, and the XRD is still mainly amorphous but has some signs of SiC formation. The thermal conductivity of the material is as low as about 0.05 W / m·K, the electromagnetic performance is improved, the dielectric imaginary part is 7.6, and the material has good wave absorption and heat insulation performance.

[0029] Example 3 SiC fiber rigid foam was prepared by doping 0.01 g aluminum nitrate nonahydrate. Specifically, 3 mL of anhydrous ethanol was mixed with 10 mL of tetrahydrofuran, 2 g of polyvinylpyrrolidone (PVP) was added to dissolve, and 1.5 g of polycarbosilane (PCS) was added, and the stirring was continued for 48 hours to obtain a precursor spinning solution. The solution was formed into an electrospinning fiber sheet under the conditions of an electrostatic spinning voltage of 10 kV, an injection speed of 5 mL / h, a collection distance of 10 cm, a needle type of G24, and a collector rotation speed of 500 rad / min. The obtained electrospinning fiber sheet was cured at 200°C for 3 hours to obtain a precursor fiber sheet, the precursor fiber sheet was cut and stacked to obtain a fiber felt with a thickness of 2 cm, and a pressure of 35 Pa was applied. The fiber felt was heat treated at 1300°C at a temperature rising rate of 8°C / min in a nitrogen atmosphere for 1 hour to complete the ceramization. The obtained SiC fiber rigid foam has a high overall porosity, XRD analysis shows a high crystallization degree, Raman spectrum shows that the degree of graphitization of the doped sample is increased, the Raman D / G intensity ratio is reduced to 1.257, the dielectric loss is reduced, and the wave absorption performance is further enhanced, reaching -9.1 dB.

[0030] Example 4 SiC fiber rigid foam was prepared by doping 0.02 g aluminum nitrate nonahydrate. Specifically, 5 mL of anhydrous ethanol was mixed with 8 mL of tetrahydrofuran, 1 g of polyvinylpyrrolidone (PVP) was added to dissolve, and 1.5 g of polycarbosilane (PCS) was added, and the stirring was continued for 36 hours to obtain a precursor spinning solution. The solution was formed into an electrospinning fiber sheet under the conditions of an electrostatic spinning voltage of 12 kV, an injection speed of 4 mL / h, a collection distance of 20 cm, a needle type of G20, and a collector rotation speed of 5 rad / min. The obtained electrospinning fiber sheet was cured at 180°C for 1 hour to obtain a precursor fiber sheet, the precursor fiber sheet was cut and stacked to obtain a fiber felt with a thickness of 4 cm, and a pressure of 40 Pa was applied. The fiber felt was heat treated at 1400°C at a temperature rising rate of 2°C / min in a nitrogen atmosphere for 3 hours to complete the ceramization. The obtained SiC fiber rigid foam has a decreased structural integrity, with phenomena such as particle aggregation and fiber roughness, and EDS shows uneven distribution of aluminum. The XRD diffraction peak intensity is weakened, and the Raman D / G intensity ratio is overall decreased, indicating an increase in carbon phase structure defects and a decrease in graphitization degree. The dielectric performance of this sample is degraded compared to the 1% doped sample, with a dielectric imaginary part of about 5.4, a decrease in porosity, but the density does not translate into performance advantages.

[0031] The compressive stress-strain curves of Examples 1-4 after high-temperature heat treatment in a nitrogen atmosphere are as follows Figure 2AL-0.5 represents 0.005 g of doped aluminum nitrate nonahydrate, AL-1 represents 0.01 g of doped aluminum nitrate nonahydrate, AL-2 represents 0.02 g of doped aluminum nitrate nonahydrate, all of which are doped aluminum nitrate nonahydrate in 10-15 mL of precursor spinning solution, from Figure 2 It can be seen from the figure that the compressive stress of the sample without doped aluminum nitrate nonahydrate is the lowest.

[0032] Best embodiment In the flask, 5 mL of anhydrous ethanol was added, and 0.01 g of aluminum nitrate nonahydrate was weighed and stirred to dissolve, forming a transparent solution containing aluminum; 8 mL of tetrahydrofuran (THF) was then added and mixed uniformly; 1 g of PVP was then added and stirred until completely dissolved; finally, 1.5 g of polycarbosilane (PCS) was slowly added, and the stirring was continued for 24 hours to obtain a uniform and stable precursor spinning solution. Compared with the electrolyte system without aluminum, the conductivity of the aluminum nitrate nonahydrate solution in this embodiment was increased to 6.73 μS / cm, the spinning current was more stable, the jet morphology was more stable, and the fiber forming property was significantly improved.

[0033] The obtained solution was filled into a syringe, and electrospinning was performed using a G20 needle. The spinning voltage was set to 12 kV, the injection rate was 4 mL / h, the distance between the spinning needle and the collection drum was 20 cm, the rotation speed of the drum was controlled at 5 rad / min, the room temperature was 25°C, and the humidity was 35%. The obtained electrospun fiber sheet was dense, continuous, and had no obvious bead defect.

[0034] The electrospun fiber sheet was placed in a hot air drying oven and heat cured at 200°C for 1 hour to obtain a precursor fiber sheet. Then it was cut into a 45 mm x 45 mm size sheet, stacked to a thickness of about 40 mm, and covered with graphite paper to apply a constant load of 40 Pa to maintain the flatness of the morphology and prevent bulging during subsequent high-temperature treatment. The sample was heat treated at 1300°C in a nitrogen atmosphere for 2 hours and then cooled to complete the ceramic conversion of the precursor. At high temperatures, aluminum elements form Al-O-C and Al-O-Si network structures, effectively enhancing the cross-linking connection between fibers, and inducing the formation of part of the graphite carbon phase, further improving the dielectric loss performance. As shown in Figure 3 , from Figure 3 (a), Figure 3 (b), Figure 3 (c), and Figure 3 (d), it can be seen that when 0.01 g of aluminum nitrate nonahydrate is doped in 10-15 mL of precursor spinning solution, the dielectric performance is the best.

[0035] The sample after heat treatment is subjected to characterization test, and the results show that the real part of the dielectric constant is 4.88, the imaginary part is 9.71, and the loss tangent is as high as 1.99 under X-band and Ku-band, indicating that the sample has good dielectric loss capacity; the thermal conductivity test shows that the thermal conductivity coefficient of the sample is 0.097 W / m·K at 100°C, and the sample has excellent thermal insulation performance; the compression strength test value is 13.932 MPa, and the sample still has good structural stability under the condition of high porosity (72.77%). The above results verify that the SiC fiber hard foam prepared by doping the aluminum-containing electrolyte for modification is superior to the undoped system in electromagnetic performance, thermophysical properties and mechanical stability, and proves the application potential and actual engineering value of the SiC fiber hard foam in the field of integrated wave-absorbing and heat-insulating protection materials.

Claims

1. A method for preparing an integrated microwave absorption and heat insulation SiC fiber rigid foam, characterized in that: Includes the following steps: Step S01: Preparation of precursor spinning solution: Mix ethanol and aluminum nitrate nonahydrate and stir until completely dissolved. Then add tetrahydrofuran and polyvinylpyrrolidone and stir until completely dissolved. Then add polycarbosilane and continue stirring to obtain a uniform precursor spinning solution. Step S02: Electrospinning: The precursor spinning solution is loaded into a syringe and electrospinned through a needle to obtain electrospinned fiber sheets. Step S03: Thermal curing treatment: Curing the obtained electrospun fiber sheet to obtain the precursor fiber sheet; Step S04: Lamination and Pressurization: Cut several precursor fiber sheets into uniform sizes, stack them to form a fiber felt of a certain thickness, cover the upper surface with a high-purity graphite plate or ceramic plate, and apply constant pressure. Step S05: Ceramicization treatment: Place the fiber felt in a nitrogen atmosphere, heat it to 1200-1500℃, keep it at the temperature and then cool it to complete the preparation of SiC fiber rigid foam.

2. The preparation method according to claim 1, characterized in that: In step S01, in every 10mL-15mL of precursor spinning solution, there are 2-5mL of ethanol, 0.005-0.02g of aluminum nitrate nonahydrate, 0.5-2g of polyvinylpyrrolidone, 1-3g of polycarbosilane, and the remainder is tetrahydrofuran.

3. The preparation method according to claim 1, characterized in that: In step S02, the electrospinning voltage is 10-16.5 kV, the injection speed is 3-5 mL / h, the collection distance is 10-20 cm, and the collector rotation speed is 5-500 rad / min.

4. The preparation method according to claim 3, characterized in that: The needle model is one of G20, G21, G22, and G24.

5. The preparation method according to claim 1, characterized in that: In step S03, the obtained electrospun fiber sheet is cured at 180-220℃ for 1-3 hours.

6. The preparation method according to claim 1, characterized in that: In step S04, a constant external pressure of 20-40 Pa is applied to the precursor fiber sheet.

7. The preparation method according to claim 1, characterized in that: In step S05, the temperature is increased at 2-8℃ / min and held for 1-3 hours.

8. An integrated SiC fiber rigid foam, characterized in that: It is prepared by the preparation method according to any one of claims 1-7.

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